In copending U.S. Application Ser. No. 541,611 filed Oct. 13, 1983, incorporated herein by reference, it is disclosed that non-metal or semi-metal fibers, such as carbon fibers, may be uniformly coated with a metal layer which is thin, continuous, and exhibits a high metal-to-core bond strength. Such metal coated fibers in the form of filaments, mats, cloths and chopped strands are disclosed therein to be useful in reinforcing metals and plastics including aluminum, steel, titanium, vinyl polymers, nylons, polyesters, etc., for use in aircraft, automobiles, office equipment, sporting equipment and other fields; and now it has been discovered that chopped lengths of such metal coated fibers, due to several inherent physical and electrical properties, are well-suited for use as chaff, i.e., dipoles or passive and active reflectors that give return readings on radar equipment, and may thus serve, e.g., as an electronic decoy.
A brief history and summary of the principles of microwave reflection by chaff is given by Butters, B. C. F., "Chaff", I.E.E. PROC., Vol. 129, Part F, No. 3, pp. 197-201 (June 1982). Dr. Butters identifies 3 principal chaff types: silver coated nylon, shredded aluminum foil, and aluminized glass. Each has disadvantages, e.g., silver coated nylon is expensive and difficult to manufacture in diameters less than about 90 microns, and shredded aluminum and aluminized glass have a comparatively high bulk density compared to silver coated nylon, have high contact resistance, and are more susceptible than silver coated nylon to distortion in manufacturing or when dispersed. However, the relatively slow rate of descent when dispersed, the high conductivity of aluminum, and the comparative ease and low cost of manufacture make aluminized glass the favored chaff material; and it is pointed out that other substrates, specifically carbon or graphite fibers, have not been successfully used for chaff because they are difficult to adherently coat with metals, and uncoated they are too resistive, to be efficient chaff.
It has now been discovered that the metal coated fibers produced in accordance with this inventor's discovery disclosed in U.S. application Ser. No. 541,611, filed Oct. 13, 1983, exhibit unexpectedly even and adherent metal coatings, and, as additionally disclosed herein, chopped lengths of such metal coated fibers are superior materials for use as chaff.
Several techniques have been developed for metal coating semi-metallic core fibers such graphite, however they have proved only marginally successful, largely due to the boundary layers present on such fibers.
High strength carbon fibers are made by heating polymeric fiber, e.g., acrylonitrile polymers of copolymers, in two stages, one to remove volatiles and carbonize and another to convert amorphous carbon into crystalline carbon. During such procedure, it is known that the carbon changes from amorphous to crystalline form, then orients into fibrils. If the fibers are stretched during the graphitization, then high strength fibers are formed. This is critical to the formation of the boundary layer, because as the crystals grow, there are formed high surface energies, as exemplified by incomplete bonds, edge-to-edge stresses, differences in morphology, and the like. It is also known that the new carbon fibrils in this form can scavenge nascent oxygen from the air, and even organic materials, to produce non-carbon layers which are firmly and chemically bonded thereto, although some can be removed by solvent treating, and there are some gaps or open spaces in the boundary layers. Not unlike the contaminants on uncleaned, unsized glass filaments, these boundary layers on carbon fibers are mainly responsible for the failure to achieve reinforcement with plastics and metals, and contribute to the high electrical resistance and poor current carrying abilities of carbon fibers as compared with metals.
Numerous unsuccessful attempts have been reported to provide such filaments, especially carbon filaments, with uniform adherent conductive coatings. Most have involved depositing layers of metals, especially nickel and copper as thin surface layers on the filaments. The metals in the prior art procedures have been vacuum deposited, electrolessly deposited, and electrolytically deposited, but the resulting composite fibers were not suitable.
Vacuum desposition, e.g., of nickel, on carbon fibers according to U.S. Pat. No. 4,132,828 (Nakamura et al.), gives an apparently continuous coating, but the vacuum deposited metal first touches the fibrils through spaces in the boundary layer, then grows outwardly like a mushroom, the coating growing away from the surface, as observed under a scanning electron microscope. The deposits are also only "line-of-sight", not penetrating to sub-surface fibers in a yarn or cloth. This is known as nodular nucleation. If the fiber is twisted, such a coating will fall off. The low density non-crystalline deposit limits use.
Electroless nickel baths have also been employed to plate such fibers, but again there is the same problem: The initial nickel or other electroless metal seeds only small spots, through holes in the boundary layer, then new metal grows up like a mushroom and joins into what appears to be a continuous coating, but it too will fall off when the fiber is twisted. The intermetallic compound is very locally nucleated, and this too limits use. In the case of both vacuum deposition and electroless deposition, the strength of the metal-to-core bond is always substantially less than that of the tensile strength of the metal deposit itself.
Finally, electroplating with nickel and other metals, to provide carbon fibers with a metal layer and achieve compatibility with metals and plastics, is reported in U.S. Pat. No. 3,622,283 (Sara). Short lengths of carbon fibers are clamped in a battery clip, immersed in an electrolyte, and by continuously reversing end on end are electroplated with nickel. When fibers produced by such a process are sharply bent, on the compression side of the bend there appear a number of transverse cracks and on the tension side of the bend the metal breaks and flakes off. If the metal coating is mechanically stripped, and the reverse side is examined under a high-power microscope, there is either no replica or at best only an incomplete replica of the fibril, the replica defined to the 40 Angstrom resolution of the scanning electron microscope. The latter two observations are strongly suggestive that failure to reinforce the matrix was due to poor bonding between the carbon and the nickel plating due to a very localized nucleation that became the site for further growth of the coating In such cases, the metal-to-core bond strength is also only a fraction of the tensile strength of the metal coating.
It has now been discovered that where electroplating is the coating technique selected, if a very high order of external voltage is applied, much higher than was thought to be achievable in the prior art, then uniform, continuous, adherent, thin metal coatings can be provided to reinforcing fibers, especially carbon fibers. The voltage must be high enough to provide energy sufficient to push the metal ions through the boundary layer to provide uniform nucleation with the fibrils directly.
Composite fibers comprising the thin and uniform metal coatings on fibers, and yarns or tows, woven cloth, and the like including such fibers prepared according to this invention, can be knotted and folded without the metal flaking off. The composite fibers can be sharply bent without producing either transverse cracking ("alligatoring") on the compression side of the bend, or breaking and flaking when the elastic limit of the metal is exceeded on the tension side of the bend. In other words, the composite fibers of the present invention are distinguishable from those of the prior art because they are continuous and the composite fibers have a thin and uniform metal coating. Additionally, the bond strength (metal-to-core) on the fibers is high. The high metal-to-core bond strengths are not critical for the suitability of the metal coated fibers of this invention for chaff, but such bond strengths are a distinction between such materials and the prior art. Metal-to-core bond strengths approaching the tensile strength of the metal can be achieved herein.
Chaff produced from such metal coated fibers has several advantages. For example, a wide range of conductive metals and combinations of such metals can be used, and coated strands can be chopped at various lengths (in relation to the operating radar frequencies the chaff is used against). In addition the metal coated fibers of the invention can be of a particular light weight, enhancing its effectiveness as chaff.